Japanese Patent No. 3106592 discloses a ballast for a discharge lamp which includes a DC power supply and an inverter circuit. The DC power supply receives an input voltage and provides a regulated DC output voltage for the inverter circuit. The inverter circuit converts the DC output voltage into a high frequency voltage to apply it to the discharge lamp for driving the discharge lamp. The DC power supply includes an inductor, a capacitor, and a switching element, and generates a predetermined DC output voltage across a smoothing capacitor by turning on and off the input voltage. The inverter circuit includes two switching elements which are connected in series across the smoothing capacitor, and generates a high frequency voltage by turning on and off the two switching elements alternately. This ballast further includes a first control unit which turns on and off the switching element of the DC power supply so as to keep the DC output voltage at a predetermined value, a second control unit which turns on and off the switching elements of the inverter circuit so as to keep the high frequency voltage from the inverter circuit at a predetermined value or change the high frequency voltage, and a driver circuit which receives a first control signal and a second control signal generated from the first control unit and the second control unit, respectively, and generates a signal for driving the switching elements.
In such a conventional ballast, the first control unit which controls the DC power supply and the second control unit which controls the inverter circuit are formed in separate integrated control circuits and are designed so that each integrated control circuit includes a driver circuit. Therefore, a circuit pattern on a printed circuit board on which these two integrated control circuits are mounted is subjected to a restriction, and mounting positions of the DC power supply and the inverter circuit, especially the switching elements used for these, have a low degree of flexibility, so, it is difficult to miniaturize the ballast circuit.